David Iluz

Microbial rhodopsins on leaf surfaces of terrestrial plants

The above-ground surfaces of terrestrial plants, the phyllosphere, comprise the main interface between the terrestrial biosphere and solar radiation. It is estimated to host up to 10(26) microbial cells that may intercept part of the photon flux impinging on the leaves. Based on 454-pyrosequencing-generated metagenome data, we report on the existence of diverse microbial rhodopsins in five distinct phyllospheres from tamarisk (Tamarix nilotica), soybean (Glycine max), Arabidopsis (Arabidopsis thaliana), clover (Trifolium repens) and rice (Oryza sativa). Our findings, for the first time describing microbial rhodopsins from non-aquatic habitats, point towards the potential coexistence of microbial rhodopsin-based phototrophy and plant chlorophyll-based photosynthesis, with the different pigments absorbing non-overlapping fractions of the light spectrum.

THE KINETICS OF THE PHOTOACCLIMATION RESPONSE OF NANNOCHLOROPSIS SP. (EUSTIGMATOPHYCEAE) : A STUDY OF CHANGES IN ULTRASTRUCTURE AND PSU DENSITY

In this study we report the kinetics of photoacclimation of the unicellular alga Nannochloropsis sp. grown under high light (HL), and subsequently transferred to low light (LL). We examined the changes in ultrastructural features, pigmentation, and photosynthetic parameters over short intervals until the LL steady state was reached. The ultrastructural changes were followed by quantitative morphometric measurements of transmission electron micrographs. We found that the increase in the relative volume of the chloroplast during acclimation to LL (twofold) was accompanied by an increase in number of stacks (twofold) and in the surface area of thylakoids per cell (2.5‐fold). The increase in photosynthetic unit (PSU) density was about 2.15‐fold. Maximal density was about 84 PSU·μm−2 in LL cells, and minimal density was 39 PSU·μm−2 in HL cells. The HL/LL ratio of the in vivo optical absorption cross‐section of PSU (σPSU) was 2.8, whereas in the in vivo optical absorption cross‐section of the cell (σcell), the trend of change was in the opposite direction: 1.7‐fold higher in LL‐acclimated cells than in HL‐acclimated cells. We propose a partial sequence of the photoacclimation processes based on our data and the derived rate constants.

Flashing light in microalgae biotechnology

Flashing light can enhance photosynthesis and improve the quality and quantity of microalgal biomass, as it can increase the products of interest by magnitudes. Therefore, the integration of flashing light effect into microalgal cultivation systems should be considered. However, microalgae require a balanced mix of the light/dark cycle for higher growth rates, and respond to light intensity differently according to the pigments acquired or lost during the growth. This review highlights recently published results on flashing light effect on microalgae and its applications in biotechnology, as well as the recently developed bioreactors designed to fulfill this effect. It also discusses how this knowledge can be applied in selecting the optimal light frequencies and intensities with specific technical properties for increasing biomass production and/or the yield of the chemicals of interest by microalgae belonging to different genera.

Adaptation of an algorithm for chlorophyll-a estimation by optical data in the oligotrophic Gulf of Eilat

Downwelling irradiance and upwelling radiance from the water surface were measured in the Gulf of Eilat (Red Sea) on 55 separate occasions in a pelagic station in the period from January 1994 to December 1996. The reflectance was calculated in seven of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) spectral channels throughout the visible spectrum. Concurrent with the optical measurement, water samples were taken to the laboratory and the concentrations of chlorophyll-a (Chl-a) were determined. The ratio of the reflectance in the blue ( R 443) to the reflectance in the green ( R 550) was regressed against the Chl-a concentration, and showed that the best-fit of the three datasets (1994, 1995 and 1996) assumed an exponential form, as found in previous work in oligotrophic waters. The coefficients determined in the current study were, however, different from those published in the literature; the reason for this probably originates in the composition of water constituents in the studied area. Small cells dominate the phytoplankton in the pelagic waters in the Gulf of Eilat, and the concentration of suspended and soluble materials of terrestrial origin is very small. The suggested algorithm enabled the prediction of Chl-a concentration within 0.08 mg m @ 3 . It was shown that calibration of the coefficients, based on empirical data, is important for increasing its prediction accuracy.

Functional changes of the visual system of the damselfish Dascyllus marginatus along its bathymetric range

Shallow-water zooplanktivorous fish rely on their vision for foraging. In shallow water, feeding efficiency decreases in dim light and thus the fish cease foraging at crepuscular hours. Creatures living in the lower parts of their depth ranges are expected to be exposed to limited light levels for longer hours. However, observations of the zooplanktivore Dascyllus marginatus showed little change in foraging duration down to 40m deep. We asked whether the visual systems functionality changes with depth along the depth range of this damselfish; we examined eye and retina anatomy for changes in visual acuity and light sensitivity and used the optomotor response to test for spatial and temporal light summation. We found only minor changes in the anatomy of the eye that are not expected to affect visual sensitivity or acuity. However, behavioural experiments showed that the deeper water fishs test performance exceeded those of fish in shallow water under lower light levels. We found that deeper water fish responded to the optomotor test at lower light levels and also had more discriminating visual acuity in low light, which can increase their potential reactive distance. The plastic adaptive ability of the visual system to low light levels may explain the fishs ability to inhabit deeper reef habitats and thus expand their depth range limits.

Continuous background light significantly increases flashing-light enhancement of photosynthesis and growth of microalgae

Under specific conditions, flashing light enhances the photosynthesis rate in comparison to continuous illumination. Here we show that a combination of flashing light and continuous background light with the same integrated photon dose as continuous or flashing light alone can be used to significantly enhance photosynthesis and increase microalgae growth. To test this hypothesis, the green microalga Dunaliella salina was exposed to three different light regimes: continuous light, flashing light, and concomitant application of both. Algal growth was compared under three different integrated light quantities; low, intermediate, and moderately high. Under the combined light regime, there was a substantial increase in all algal growth parameters, with an enhanced photosynthesis rate, within 3days. Our strategy demonstrates a hitherto undescribed significant increase in photosynthesis and algal growth rates, which is beyond the increase by flashing light alone.

Optical habitats of ultraphytoplankton groups in the Gulf of Eilat (Aqaba), Northern Red Sea

Solar light penetrates deep into the clear water of oligotrophic oceans and may have both beneficial and destructive effects on marine phytoplankton. In oligotrophic waters worldwide, phytoplankton communities consist mostly of ultraphytoplankton of the groups Synechococcus and Prochlorococcus, which differ in their optical properties and, therefore, are better suited for thriving in different niches along the natural vertical light gradient. In this article, we aim to draw the optical boundaries separating the two populations in order to get a better insight into the light-driven dynamics in ultraphytoplankton-community structure and to predict future trends. We report spectral, photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) penetration through the stratified season along with temporal and vertical distributions of Synechococcus and Prochlorococcus in the Gulf of Eilat (Aqaba). These light-field parameters are used to define the apparent limits of the vertical distribution of Synechococcus and Prochlorococcus throughout the water column. Furthermore, we formulate the necessary empirical algorithms, allowing for characterization of the optical habitats defined in this study by remote-sensed or in situ radiometric measurements.

Flashing light enhancement of photosynthesis and growth occurs when photochemistry and photoprotection are balanced in Dunaliella salina

Photosynthesis and growth in Dunaliella salina were significantly (P ≤ 0.01) enhanced under flashing light with a light–dark cycle tailored for the minimal activation of the violaxanthin cycle (VC), in comparison with cultures grown under constant illumination of the same time-integrated photon dose. The experiments were performed without using inhibitors or any other experimental conditions other than light. The flashing light (FL) was optimally balanced for the growth and photosynthesis of D. salina, which was characterized by a decreased VC de-epoxidation rate. The high-light induction of non-photochemical quenching (NPQ) was significantly slower in the FL-acclimated cells compared with those acclimated to continuous illumination. The comparative investigation of the cell ultrastructure revealed distinct effects of high-light stress in cultures grown under continuous light; this was not the case under FL. We conclude that a slower activation of the VC takes place under balanced flashing light, when each flash is followed by a dark period long enough for utilization of the photons absorbed during the flash at a given irradiance. The activation kinetics of the VC and, hence, the induction of NPQ seem to play an important role in the enhancement of the photosynthesis observed in D. salina grown under flashing light. Potential use of these parameters for optimization of FL for microalgal cultivation is discussed.

Blocking of Pseudomonas aeruginosa and Chromobacterium violaceum lectins by diverse mammalian milks.

Pseudomonas aeruginosa and Chromobacterium violaceum morbid and mortal infections are initiated by bacterial adherence to host-cell receptors via their adhesins, including lectins (which also contribute to bacterial biofilm formation). Pseudomonas aeruginosa produces a galactophilic lectin, PA-IL (LecA), and a fucophilic (Lewis-specific) lectin, PA-IIL (LecB), and C. violaceum produces a fucophilic (H-specific) lectin, CV-IIL. The antibiotic resistance of these bacteria prompted the search for glycosylated receptor-mimicking compounds that would function as glycodecoys for blocking lectin attachment to human cell receptors. Lectins PA-IL and PA-IIL have been shown to be useful for such glycodecoy probing, clearly differentiating between human and cow milks. This article describes their usage, together with CV-IIL and the plant lectin concanavalin A, for comparing the anti-lectin-dependent adhesion potential of diverse mammalian milks. The results show that the diverse milks differ in blocking (hemagglutination inhibition) and differential binding (Western blots) of these lectins. Human milk most strongly inhibited the 3 bacterial lectins (with PA-IIL superiority), followed by alpaca, giraffe, and monkey milks, whereas cow milk was a weak inhibitor. Lectin PA-IL was inhibited strongly by human, followed by alpaca, mare, giraffe, buffalo, and monkey milks, weakly by camel milk, and not at all by rabbit milk. Lectins PA-IIL and CV-IIL were also most sensitive to human milk, followed by alpaca, monkey, giraffe, rabbit, and camel milks but negligibly sensitive to buffalo and mare milks. Plant lectin concanavalinA, which was used as the reference, differed from them in that it was much less sensitive to human milk and was equally as sensitive to cow milk. These results have provided important information on the anti-lectin-dependent adhesion potential of the diverse milks examined. They showed that human followed by alpaca, giraffe, and Rhesus monkey milks efficiently blocked the binding of both the galactophilic and fucophilic (>mannophilic) pathogen lectins. The results also proved the advantage of isolated pathogenic bacterial lectins as superb probes for unveiling bacterial adhesion-blocking glycodecoys. The chosen milks or their polymeric glycans might be implicated in blocking lectin-dependent adhesion of antibiotic-resistant pathogens leading to skin, eye, ear, and gastrointestinal infections.

Quantum Yields in Aquatic Photosynthesis

Primary productivity is the product of the light energy absorbed by plants and the efficiency by which this energy is stored as a photosynthate. The quantum yield (φ) of photosynthesis is defined (Eq. 1) as the molar ratio between oxygen released in photosynthesis (or carbon assimilated) to photons absorbed in the process (Fig. 1) (Dubinsky, 1980; Dubinsky & Berman, 1976, 1979, 1981; Dubinsky et al., 1984). The quantum yield is, therefore, equal to the ratio of photosynthetically stored radiation (PSR) to the energy absorbed photosynthetically usable radiation (PUR)[for definitions, see Morel (1978)].